1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system. More particularly, the present invention relates to an apparatus and method for canceling a Direct Current (DC) offset in a receiver of an Orthogonal Frequency Division Multiplexing (OFDM) system that is mainly used in a BWA communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) enables a faster transmission using a relatively less frequency band and known to be robust in a multiple path channel environment. As such, OFDM is more widely employed in recent communication systems such as wireless Local Area Network (LAN), Wideband Broadband (Wibro), etc.
A receiver of an OFDM communication system demodulates a signal received from an antenna, via a Radio Frequency (RF) end, and then restores the signal to an original state. A signal received from an antenna goes under a frequency down-conversion and a signal-magnitude amplification, so that a receiver can acquire a desired signal. To this end, several types of analog devices such as a mixer and an amplifier are used. These devices meet the standards to some degree, but input signals are distorted due to drawbacks in inter-device insulating properties and orthogonality. One of main distortion causes is a DC offset.
The DC offset is generated because an RF of an OFDM wireless communication system is self-mixed in an analog circuit or because differential baseband devices are mismatched. The DC offset may deteriorate reception performance in instances of mixing an unexpected signal at a baseband end and receiving the baseband signal at a receiver, that is, during the process of inputting a signal to an Analog-to-Digital Converter (ADC).
In modern receivers, a direct conversion method is mostly used. The direct conversion method is a method in which an RF end directly converts to a baseband frequency. As the receiver using the direct conversion method directly converts a received RF signal to a baseband signal without converting the RF signal to an Intermediate Frequency (IF) signal, a filter, a mixer, an amplifier, etc. used for converting to an IF signal can be omitted. However, because a received RF signal and a local oscillation signal have the same frequency, a DC offset may be generated more, thus there is a need for canceling the DC offset.
At present, there are several technologies for correcting a DC offset existing within a Radio Frequency Integrated Circuit (RFIC). However, in most technologies, an RFIC measures and corrects its own DC offset, thus a modulator/demodulator (modem) fails know exactly the magnitude of a DC offset of its substantially received signal. As a result, if there is a DC offset in a device within an RFIC, the DC offset may not be canceled or fails to be reduced. There is a technology for estimating and correcting a DC offset in real time during a predetermined period of time irrespective of synchronization. However, this method has a problem that it may cause a rather higher error in an OFDM system. To address this, there is also a method of canceling a DC offset using a preamble only after the synchronization. However, there is a problem that, if DC offset correction is performed only after the synchronization acquisition, it is difficult to acquire an initial synchronization when there is a high DC offset in a non-corrected signal.
If a receiver attempts to acquire an initial synchronization, gains of amplifiers within the receiver increase at a low signal level, hence even a DC offset increases. If there is a high DC offset in the input signal of a modem, there is a problem of synchronization acquisition being increased only at a strong electric field and being decreased at a weak electric field if it is intended to reduce a gain within an RFIC and reduce a magnitude of a DC offset. Further, there is a problem of causing a deterioration in the reception sensitivity of a receiver that can acquire an initial synchronization and a decrease of a dynamic range of a reception unit of a terminal.
FIG. 1 is a graph illustrating a conventional process of DC offset variation of an RFIC. As shown, upon power on, most RFICs perform the initialization and its own calibration. Because the self-calibration is implemented by the RFIC, a modem is unaware of the magnitude of a substantially received DC offset. As a result, if there is a DC offset in a device within the RFIC, the DC offset may not be canceled or fails to be reduced to a desired level.
Also, although calibration is performed (i.e., although synchronization is achieved), an RFIC performs reception/transmission (Rx/Tx) switching. Thus, if temperature changes as time goes by, it cannot be calibrated with an initial calibration value. Further, there is a problem in that, if a Mobile Station (MS) in a sleep state or idle state for a lengthy time powers on and attempts a synchronization acquisition, a proper synchronization acquisition cannot be achieved using an initial value.